During the processing of an electronic chip a semiconductor wafer is exposed to a radiation source in order to develop a photoresist layer on top of the wafer. A mask is used between the source and the wafer to selectively block the radiation and, thereby, to develop a desired design in the photoresist which controls subsequent etching of the chip. At various processing stages different masks may be used to develop different desired designs on the wafer. It is essential that the various masks be correctly aligned with the wafer so that tight tolerances, allowing, for example, the fabrication of sub-micron width lines, may be maintained.
Optical aligners have been used in the prior art to manually align wafers and masks. Prior art optical aligners have been slow and subject to operator error because of the need to visually align reference marks lying in different planes. Other techniques using Fresnel lenses or diffraction gratings have been proposed but have proved to be adversely sensitive to reference mark variations and have not allowed dynamic control during wafer exposure.
In accordance with the illustrated preferred embodiment of the present invention, alignment is accomplished by detecting a differential capacitance between ridges located on the wafer and sets of interdigitated fingers located on the mask. An electrical signal applied to the wafer is capacitively coupled from the wafer ridges to the overlying mask fingers. The wafer and the mask are aligned when the coupled signals observed on each set of interdigitated fingers are equal in amplitude. Since signal coupling occurs between numerous fingers and ridges, errors due to variations in the fabrication of individual fingers or ridges are averaged if the finger and ridge repetition patterns (constant, chirped or random) are kept substantially identical. Further, rotational or orthogonal alignment may be achieved with the use of multiple finger/ridge sets and alignment may be automated by using the measured signals to control positioning equipment.
In accordance with another preferred embodiment of the present invention, conductive shields are placed between the wafer and interconnecting side lines of the fingers. The shields ensure that coupling only occurs between the ridges and the fingers and not between the ridges and the side lines so that lateral movement of the wafer does not cause variations in the differential capacitance to occur. This allows independent single-dimension alignments to be made.
In accordance with an additional preferred embodiment of the present invention, two sets of fingers are driven by a single driving signal. A phase shifter shifts the driving signal applied to one finger set so that the signal on the finger set is 180 degrees out of phase with the signal on the other finger set. A voltmeter detects a coupled signal on the wafer and a null is observed when alignment is achieved.
In accordance with a further preferred embodiment of the present invention, alignment is achieved with the wafer grounded. A single driving signal is applied to two finger sets through a transformer and a diode bridge and is coupled to ground through the wafer. A recharge path is provided through conductive shields overlaying the finger sets. Alignment is achieved when a D.C. current null is observed.
In accordance with still another preferred embodiment of the present invention, alignment is performed on a mask and a wafer which are separated by a substantial distance as required in some optical lithography or metrology applications. In these applications the distance between the mask and the wafer is too great for meaningful capacitive coupling to occur between the mask and the wafer. Two groups of references, one for the mask and the other for the wafer, are attached to a backbone frame and are initially aligned together. The mask is aligned to one reference and the wafer is aligned to the other reference by measurement of the differential capacitances involved.